Four cytogenetic techniques were compared to detect the clastogenicity of Mitomycin C in somatic or germinal cells of Rhesus monkeys (Macaca mulatta) in vivo: peripheral lymphocyte and bone-marrow cytogenetics, the micronucleus test in bone-marrow, cytogenetic evaluation of primary spermatocytes. Only the lymphocyte test failed to detect clastogenicity probably due to absence of DNA replication in these cells during treatment. The micronucleus test also failed in samples collected by biopsy. The bone marrow test proved to be as sensitive as the micronucleus test and could be performed with biopsy material. The lymphocyte test may also be potentially useful with testing of chemicals that are not dependent on DNA replication for their action. The cytogenetic test of primary spermatocytes was positive only when treatment was applied during the pre-meiotic S-phase. The pre-meiotic S-phase is 17 days before diakinesis-metaphase I.